Hydraulic valves are used in a variety of applications, such as the control of hydraulic cylinders. In particular, such valves are an integral part of the control system for hydraulically operated arms of heavy machinery such as backhoes, excavators, and the like. Such a valve generally has an internal valve chamber with ports that allow fluid to flow into and from the internal chamber, for example, a pressure port through which pressurized fluid flows into the valve chamber, one or more load ports through which the pressurized fluid is conducted to a load-moving device such as a hydraulic cylinder, and one or more exhaust ports for depressurizing the valve chamber. A movable valve member within the valve chamber regulates communication between the pressure, load and exhaust ports.
Among hydraulic valves, dual spool or double cylinder valves are well known. See in particular Anderson U.S. Pat. No. 4,537,220, issued Aug. 27, 1985, Hoffman U.S. Pat. No. 3,630,234, issued Dec. 28, 1971, Martin, U.S. Pat. No. 4,201,116, issued May 6, 1980, Taplin U.S. Pat. No. 4,456,031, issued Jun. 16, 1984 and Garnjost et. al. U.S. Pat. No. 4,338,965, issued Jul. 13, 1982.
In many known valves, the valve is actuated manually by a handle, as in Petry U.S. Pat. No. 3,472,264, issued Oct. 16, 1969 and Woodcock U.S. Pat. No. 4,134,418, issued Jan. 16, 1979. Byers U.S. Pat. No. 3,636,978, issued Jan. 25, 1972, and McKay U.S. Pat. No. 4,606,369, issued Aug. 19, 1986 describe systems wherein the handle provides tactile feedback, i.e., a resistance that provides a feel of the valve pressure to the operator.
Despite the availability of a wide variety of hydraulic valves, control systems utilizing such valves for operating hydraulic digging arms and the like lack a means for allowing the operator to feel, through the handle, a force proportional to the hydraulic pressure. Such a force would warn the operator when an obstruction has been encountered. For example, when excavating near a foundation, the excavator operator normally cannot readily tell whether the digging arm has hit the foundation wall, possibly damaging it. Tactile feedback can prevent such accidents by allowing the operator to feel the sudden increase in pressure when the obstruction is encountered. Tactile feedback in the form of a variable resistance control handle, however, would create a problem in situations where the operator might want to lift and hold a load without forcefully holding down the control handle. The present invention addresses this problem by providing a neutral-centering valve construction suitable for control of hydraulic-cylinder operated devices.